1. Field of the Invention
The present invention relates to an optical position measuring system for detecting the relative position of two objects which are movable with respect to each other.
2. Description of Related Art
An optical position measuring system in accordance with the species is known from EP 0 223 009 B1 of Dr. Johannes Heidenhain GmbH and will be explained below by means of the schematic representations in FIGS. 1a to 1c. A scale graduation M in the position measuring system is scanned by a scanning unit A, which is arranged parallel with the above mentioned grating M and is displaceable in the x direction. In the process, signals, which have been modulated as a function of the displacement, are detected with the aid of the scanning unit A and are further processed by means of an evaluation unit, not represented.
Besides a scanning plate AP, the scanning unit A contains a optical condenser element K and a support plate T; the individual components of the scanning unit A and their functions will be described in detail in what follows. The scanning plate AP has at least two scanning fields AF1, AF2, which are displaced in respect to each other in the measuring direction x by a fraction of the graduation period TP, i.e. the scanning fields AF1 and AF2 are arranged offset from each other in the measuring direction x by the distance D; both the scanning fields AF1 and AF2 as well as the graduation M have only been indicated schematically. The scanning fields AF1, AF2 on the scanning plate AP are preferably designed as phase gratings, by means of which phase-shifted partial light beams of different orders of diffraction can be generated by each one of the two scanning fields AF1, AF2 by means of a ratio, different from 1:1, between the step width to the groove width. Two of the resultant orders of diffraction are used per scanning field AF1, AF2, preferably the +1st and the -1st orders of diffraction. Therefore a total of four partial signals results, which can be further processed. Here the grating parameters have been selected such that a phase shift of 90.degree. results between the partial signals of the +1st and -1st order of diffraction. For reasons of clarity the representation of the light beam path is omitted.
The various partial light beams are detected via downstream-connected detector elements D11, D12, D21 and D22, which are arranged on the support plate T of the scanning unit A; here, FIG. 1b shows the relative arrangement of the various detector elements D11, D12, D21 and D22 in the detector plane, as well as the light source L arranged on the support plate T.
An optical condenser element K is moreover arranged between the support plate T and the scanning plate AP, which is used for the collimation of the light beams emitted from the light source L as well as for showing the partial light beams reflected in the direction of the detector elements D11, D12, D21 and D22. Therefore the support plate T with the detector elements D11, D12, D21 and D22 and the light source L is arranged in the focal plane of the optical condenser element K within the scanning unit A.
For detecting the four partial signals, it is necessary to separate them spatially. In this case the spatial separation of partial signals phase-shifted by 90.degree. is already made possible by splitting the partial light beams of each scanning field AF1, AF2 into different order of diffraction, i.e. for detection, the +1st and -1st order of diffraction used of each scanning field AF1, AF2 are provided spatially separated. These are two partial signals, shifted by 90.degree., from the scanning field AF1, which are detected by the detector elements D11 and D12; the partial signals, also phase-shifted by 90.degree., generated via the scanning field AF2 are detected by two further detector elements D21, D22. In this case the splitting into different orders of diffraction takes place in the drawing plane of FIG. 1a. The partial signals from the two scanning fields AF1 and AF2 must still be spatially separated. To this end it is proposed in EP 0 223 009 to assign selectively acting means AE1 and AE2 to the scanning fields AF1 and AF2 arranged in an offset manner. A spatial splitting perpendicularly in respect to the drawing plane of the two pairs of partial signals originating in the scanning fields AF1 and AF2 is caused by means of this.
The spatial conditions become clear in particular in the representation in accordance with FIG. 1b, in which furthermore the phase relations between the individual partial signals have also been drawn. The two left detector elements D11 and D12 are used for detecting the partial light beams of the +1st and -1st order of diffraction resulting from the scanning field AF1, the partial light beams diffracted into the +1st and -1st order of the scanning field AF2 are detected via the two right detector elements D21 and D22.
The phase shift of 180.degree. between the partial light beams resulting from the scanning fields AF1 and AF2 is caused by the mentioned mutual offset D of these scanning fields AF1 and AF2 in the measuring direction x. The corresponding offset D in this case is D=TP/2*(N+1/2), wherein TP represents the graduation period of the scale graduation, and N=0, 1, 2 . . . applies. In order to generate the two desired output signals 0.degree., 90.degree., phase-shifted by 90.degree., for further processing from the four different partial signals in the end, it is now proposed in EP 0 223 009 to wire the detector elements D11, D12, D21, D22 in the manner represented in FIG. 1c. This means that for forming the 0.degree. signal, the two detector elements D11 and D21 with partial signals phase-shifted by 180.degree. are wired anti-parallel in relation to each other, while for forming the 90.degree. signal, the two detector elements D12 and D22 with opposite-phase partial signals applied to them, are also wired anti-parallel in relation to each other. Accordingly, respectively those detector elements D11, D12, D21, D22 are wired anti-parallel in relation to each other, to which opposite-phase signals are applied, wherein the opposite-phase partial signals always come from different scanning fields AF1, AF2.
In summation it should be emphasized that the 90.degree. phase shift between the partial signals of the +1st and -1st order of diffraction is set by the selection of the grating parameters of the scanning fields AF1, AF2, while the respective opposite-phases partial signal with a 180.degree. phase shift in relation to this comes from the respectively other scanning field, which is arranged offset by the distance D in relation to the first one in the measuring direction x.
The position measuring system proposed in EP 0233 009, however, has certain disadvantages. For example, the counter-clock signal formation takes place in that partial signals from different scanning fields AF1, AF2 are switched anti-parallel. In case of local degradation of the scanning fields AF1, AF2, the result of this are so-called scanning ratio errors, because the amplitudes of the anti-parallel switched counter-phase partial signals from the different scanning fields AF1, AF2 are not identical. Such errors result, inter alia, not only in the interferential measuring systems described in EP 0 223 009.
Based on the outlay of the phase shift of the interference signals of the +1st and -1st order of diffraction from the two scanning fields AF1, AF2 by 90.degree., the degree of modulation of these signals is greatly diminished in contrast to a dimensioning of the phase shift by 120.degree.. An optimal modulation of the interference signals is to be expected in an interferential three grating sensor with an outlay of the grating parameters of the scanning fields with a phase shift of 120.degree. between the orders of diffraction employed. This has been achieved, for example, in a position measuring system which has been described in EP 0 163 362.
Thus, clearly higher demands are made on the grating production with a phase layout of 90.degree. between the interference signals of the +1st and -1st order of diffraction. If the same production tolerances during the grating production as in the case of a 120.degree. phase shift would be allowed in the phase layout to 90.degree. between the interference signals of the +1st and -1st order of diffraction, considerably higher fluctuation in the degree of modulation and phase angle of the partial signals would have to be expected.